This paper summarizes the structural materials used in light water reactors (LWR), their historical degradation evolution vs. time, improvements and subsequent vulnerabilities discovered, and possible future degradation phenomena over long-term operation. Little was known about high-temperature water environments 50+ years ago, and reasonable but inadequate judgments were made on structural materials based partly on simple, short-term tests and partly on intuition of low-temperature corrosion phenomena. A steady evolution and improvement in structural materials occurred, especially over the first 20 years of LWR experience. Degradation continues and is managed by a combination of inspection, mitigation, and replacement, although concerns for surprises remain because of declining expertise, funding, and historical conservatism. Materials of higher resistance undoubtedly exist, and some examples are discussed. New forms of degradation and aging synergies that accelerate the kinetics of known degradation phenomena are considered.

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